In recent years, automatic focus or autofocus (AF) and automatic exposure (AE) video and still photographic cameras have come into common usage as integrated circuits, photosensors, e.g. linear CCD arrays, and miniaturized servo elements have decreased in cost and become more available, thereby allowing for systems capable of subject/object range finding used for the correction of taking lens focus positioning. In addition, the availability of inexpensive microprocessors, RAM and ROM chips, and other micro-controller components has allowed designers to incorporate sophisticated algorithms with active and/or passive range sensors for AF systems and with multi-segmented photometric sensor arrays for AE systems to provide the user with highly accurate "point and shoot" capabilities. These capabilities have been incorporated in relatively simple and inexpensive view finder photographic cameras and in highly complex and expensive single lens reflex (SLR) photographic still cameras using through-the-lens (TTL) optical systems, as well as in video cameras and in electronic still cameras.
In AF control systems, active and/or passive range finding AF systems have been developed to provide signals from which the distance between the camera imaging or taking lens and (typically) the subject or object (hereafter object) in the scene that the user has centered the imaging lens on can be determined and employed with a motorized imaging lens adjustment to adjust the position of the imaging lens for best image focus for the object distance.
One such range finding AF system employs one or more pairs of linear photosensor element arrays, e.g. CCD or photo diode linear arrays, that are positioned laterally in the same plane a fixed distance apart, which forms the baseline of the autoranging triangulation system, also referred to as a "parallax" AF system. In parallax AF systems, the baseline is a necessary dimension which allows for the formation of similar triangles as a deterministic means of computing object distance. Generally, as the AF baseline dimension and focal length increases and the linear sensor array width (pitch) decreases, the maximum sensing distance increases.
Typically, in the parallax range finding AF system, an array focusing lens is positioned with respect to each of a pair of separated co-planar, linear photosensor arrays on which the object image in SLR camera systems are focused. The array focusing lenses are arranged symmetrically with respect to the optical axis of the camera's imaging lens, and light passing through the lens (TTL) is diverted by a half silvered mirror through the pair of focusing lenses and onto the co-planar, linear arrays, e.g. as shown in U.S. Pat. Nos. 4,643,557 and 4,697,905. In such TTL AF systems, the imaging and taking lens is motor driven and the AF calculations are conducted during lens focus adjustment.
In non-TTL, or viewfinder, point and shoot, cameras, the pair of array focusing microlenses or lenslets and the associated pair of linear arrays of photosensitive elements are mounted on the camera a distance away from the taking lens. Typically, with equal number, photosensitive element arrays, both the pair of array focusing lenses and the taking lens are commonly motor driven in the adjustment of the lens focus.
FIG. 1 schematically illustrates the arrangement of these parallax AF components in a simplified fashion in a point and shoot camera. An object is imaged by a camera taking lens 12 in a lens-to-object cone 10 (representing the projection of a point on the object as projected on the aperture of the taking lens 12) onto a film plane 14 when the camera is aimed at the object. A pair of AF lenslets or microlenses 20, 22 are mounted in a microlens and linear array assembly 24 in alignment with the pair of photosensor or CCD linear arrays 30, 32 to focus object images thereon from the same portion of the object field. In the system of FIG. 1, a field lens (not shown, but corresponding to field lens 16 of FIGS. 3 and 4) conjugate to the microlenses 20, 22 may also be required to ensure that the microlenses 20, 22 are viewing similar parts of the object, so that the parallax computation will entail the required equivalent image as recorded on each sensor array. The CCD linear arrays 30, 32 are fixed in position, while the taking lens 12 is movable along its optical axis by a motor driven gear or other suitable lens adjustment mechanism 34 to adjunct the camera lens-to-object distance S.
As indicated above, the parallax, .tau., of the object images as focused on the two linear CCD arrays 30, 32 of the microlens and linear array assembly 24 is computed by autocorrelation of the signal sets d1, d2 output from the pair of CCD linear arrays 30, 32 to the micro-controller 36. The signal sets d1, d2 are developed by read-out of the linear CCD array elements, that have recorded equivalent object images, by the micro-controller 36. The computed autocorrelation lag coefficient .tau. then yields a direct measure of the relative displacement (parallax .tau.) of the object image at the camera from which the camera taking lens-to-object distance, S is calculated. A fundamental equation relating the measured displacement to a required change in imaging lens 12 position to achieve the position of best focus is used to deterministically compute the correct position for taking lens 12 and to derive a lens motor drive signal in the micro-controller 36. A lens focus drive motor 38 is energized by the motor drive signal which adjusts the lens adjustment mechanism 34 to drive the imaging or taking lens 12 to the best focus position.
The operation of a typical passive AF system of this type incorporated into an SLR camera using TTL optics is described in the above-referenced '557 patent to Ishizaki et al (incorporated by reference herein in its entirety). In the TTL environment, the object image is transmitted to the microlenses through the camera taking lens and a field lens.
Passive AF systems employing parallax suffer from signal-to-noise problems in low ambient light conditions resulting in use of active range finding systems where a light beam, e.g. a laser or LED or infrared light beam, is emitted by the camera and directed onto the object in the scene to be focused on. The reflected light is imaged on the AF system linear arrays, and the camera to object distance is calculated by the parallax methods. Such a combined active and passive AF system is disclosed in U.S. Pat. No. 4,818,865, wherein the active AF system LED emission is invoked automatically at low ambient light levels.
A contrast AF system for a point and shoot camera is depicted in FIG. 2 wherein the object is imaged by a single AF lens 26 onto the two photosensor arrays 30, 32 positioned in spaced apart (along the optical axis), parallel planes. The contrast AF system determines object best focus position based on the differential intensity in terms of variance (RMS squared) or integrated intensity as recorded on the two photosensitive arrays.
As shown in FIG. 2, the single AF lens 26 has an optimal focus that is optically conjugate along the broken line defining the sensor image plane 28 with the camera film plane 14. A key feature of the image sharpness or contrast AF system is that the linear arrays 30, 32 are now displaced with respect to each other along the optical axis of the single lens element 26 that forms the object image in the sensor image plane 28 by a distance .DELTA.d. The single AF lens 26 is mechanically coupled to move with the lens adjustment mechanism 34 for adjusting the imaging lens 12, while the linear arrays 30, 32 are fixed in their separate planes. In a TTL camera, the taking lens replaces the single AF lens 26 of FIG. 2, and the sensor plane (bisecting the distance between the two spaced apart sensor arrays) is conjugate to the image or film plane of the taking lens.
When the taking lens 12 is in focus for an object associated with the lens-to-object cone 10, for say the camera lens-to-object distance S, the single AF lens 26 will have been set to provide the object's image ideally focussed at the sensor image plane 28. Therefore relatively blurred object images that are equally de-focussed are also imaged at each of the linear arrays 30 and 32. In this case, the blur circles due to de-focus will be equivalent in spread and result in similar de-focussed image blur patterns which result in corresponding sets of electrical output signal amplitude patterns that are essentially equal in intensity and shape. A spatial correlation of the two sets of signal patterns produced by the two linear arrays 30, 32 may be made in the micro-controller 36. The peak magnitude obtained from correlation of the two de-focussed sensor images will be maximal for the condition of FIG. 2 where the plane of best focus for the object at S is midway between the planes of the sensor arrays 30, 32. This "peak correlation" condition is due to the principle that the reduction of image intensity recorded by either of the arrays will fall at a faster rate as de-focus error increases than the rate at which the intensity rises as best focus is approached. Thus, as the object distance varies from the S location, the intensity on the array 30 or 32 closer to object focus will increase, but at a slower rate than the rate of intensity decrease for the array farther from object best focus.
Thus, in principle, the AF sensor image peak correlation will only be maximal when the object is focused by the AF microlens 26 in the sensor image reference plane 28 midway between the planes of the linear arrays 30, 32. This reference plane 28 position of maximum peak correlation is calibrated to the film plane 14. When the object is focussed at sensor reference plane 28, the resulting maximal peak correlation of the sensor array images indicates that the taking lens is at the best position for objects at lens-to-object distance S. A maximal peak correlation result for an arbitrary taking lens 12 position indicates that the object is at the best focus position, and the image of the object is in focus at the film plane 14.
If the actual camera lens-to-object distance S should change, the peak correlation will decrease also indicating that the taking lens 12 is not focussed at the film plane 14. The linear array 30 or 32 with the greatest intensity output indicates the direction of focus shift and the direction that the taking lens 12 should move to regain best film plane focus. When the AF drive motor 38 is thereby commanded to move the taking lens 12 in the correct direction, the taking lens 12 and the AF lens (or the linear arrays 30, 32) are driven relative to the object, thereby resulting in an increase in the maximum peak correlation of patterns of the signal sets provided by the linear arrays 30, 32 to the micro-controller 36. When the peak correlation has again been reached, the AF drive motor 38 is de-energized and the taking lens 12 and single AF lens element 26 cease movement.